This invention relates to the hydroformylation of a lower alkene to produce an aldehyde having one carbon atom more than the alkene feedstock. The hydroformylation process comprises reacting a mixture of hydrogen and carbon monoxide with the alkene in the presence of a catalyst which comprises rhodium complexed with a triorganophosphine as exemplified by triphenylphosphine. The reaction is conducted in the presence of liquid reaction medium comprising a high-boiling solvent.
The invention is specifically directed to those reaction systems, which are well known in the pertinent prior art in which the hydrogen, carbon monoxide, and vapors of the alkene are sparged through the liquid reaction medium during the course of the reaction both to effect agitation of the reaction medium and also to strip out the aldehyde product as it is formed, in the vaporous mixture exiting the hydroformylation reactor.
The literature dealing with hydroformylation reaction systems of the type just described is voluminous. U.S. Pat. No. 3,527,809, to Pruett et al, provides a comprehensive discussion of the chemistry of these systems. U.S. Pat. No. 3,239,566, to Slaugh et al, is also pertinent as background for the basic process.
U.S. Pat. No. 4,151,209, to Paul et al, provides a discussion of product recovery by stripping of the liquid reaction medium and also discusses applicable high-boing reaction solvents. U.S. Pat. No. 4,480,138, to Hackman et al, also discusses especially useful reaction solvents and deals at considerable length with the technology of the product-stripping operation itself.
U.S. Pat. No. 4,148,830, to Pruett et al, teaches the use of high-boiling reaction by-products as the reaction solvent.
The recovery of the aldehyde product from the liquid reaction medium can be effected in a number of ways including continuously drawing off a slip stream from the hydroformylation reactor and distilling it to separate relatively low-boiling compounds including the aldehyde product as an overhead stream and then returning the stripped residue to the hydroformylation reactor as desired. Alternatively, the continuous stripping of the reaction medium by sparging hydrogen, carbon monoxide, and alkene vapors into the liquid contained in the hydroformylation reactor, with the aldehyde product being continuously withdrawn from the top of the reactor in the exiting gases, is particularly useful when the aldehyde is of substantial volatility, as in the case of propionaldehyde and the butyraldehydes. The continuous hydroformylation of propylene in a gas-sparged reactor in this manner is discussed by Hershman et al in Industrial and Engineering Chemistry Product Research and Development, Vol. 8 (1969), pages 372-375.
U.S. Pat. No. 4,247,486, to Brewester et al, describes a system in which the gas recycle through the hydroformylation reactor is controlled in such a manner as to maintain the liquid level in the reactor and control the build-up of high molecular weight by-products.
The potential problem of loss of valuable rhodium in the reactor overhead in the form of entrainment is mentioned in U.S. Pat. No. 4,247,486, and also in U.S. Pat. No. 4,287,369, to Harris et al, who also describe a product-recovery system in which the gases exiting the hydroformylation reactor are subjected to condensation with non-condensed components being recycled to the hydroformylation reactor. Both U.S. Pat. No. 4,247,486 and U.S. Pat. No. 4,286,369 disclose the use of demisting pads through which the gases exiting the hydroformylation reactor are passed in order to remove entrained liquid droplets for return to the reactor. In neither case is there a suggestion that the demisting pads are in any way deficient in preventing loss of rhodium from the reaction system.
However, as disclosed in U.S. Pat. No. 4,613,701 to Strong, conventional entrainment separators such as demisting pads are not completely effective in preventing loss of rhodium into the reactor overhead system. Even small losses of rhodium through the entrainment-separation system are very significant economically because of the high cost of rhodium. Also, of course, rhodium is a strategically important metal regardless of its monetary cost. It is not known with certainty whether these loses of rhodium through the, for example, demisting pads occur as very fine entrainment which is not trapped by the pads or whether there is actually some volatilization of the rhodium as complexes which have an appreciable although certainly very low, volatility.
In accordance with the invention as disclosed in U.S. Pat. No. 4,613,701, the vapors which are stripped overhead with the aldehyde products in a hydroformylation reactor operating as previously described are condensed and then distilled to produce, a first distillate comprising the aldehyde hydroformylation product along with a first residue comprising substantially the entirety of those components of the hydroformylation product condensate which are less volatile than the aldehyde product. This first residue is then redistilled to separate it into (a) a redistillation overhead stream which comprises substantially the entirety of those compounds present which are more volatile than the ligand which is used in the hydroformylation catalyst system (typically and normally triphenylphosphine) and (b) a redistillation residue which comprises those components of the initially-obtained hydroformylation reaction product condensate which are equal to or lower than the ligand (typically triphenylphosphine) in volatility and which include any rhodium moiety which may have been initially present in the condensed hydroformylation reaction product vapors. The rhodium-containing residue from the second distillation not only contains substantially the entirety of the rhodium which was initially lost out the top of the hydroformylation reactor, but it also is substantially free of high-boiling reaction by-products which tend to deactivate the hydroformylation catalyst as they build up in the hydroformylation reactor. Free of these compounds, the residue obtained from the second distillation is suitable for recycle to the hydroformylation reactor as catalyst makeup.
Unfortunately, as recognized by Strong there still remains a small portion of aldehyde product in the first residue which is separated from the bulk of the aldehyde product. For certain aldehydes such as butyraldehyde this requires that the first residue which contains small quantities of butyraldehyde be distilled in two stages, a low temperature stage which avoids a temperature above 230.degree. C. which is the auto-ignition point of butyraldehyde and a second high temperature stage to remove the other high-boiling heavy ends from the catalyst makeup residue. This "two-pass" operation needed to remove explosive aldehydes is cumbersome and not economical. It would be advantageous to recover the catalyst makeup residue in a single distillation and yet still run the distillation safely without the dangers of ignition of low boiling aldehyde products such as butyraldehyde. Such operation is the subject of this invention and forms its primary objective.